Mercedes-Benz CLS: Oil cooler seal leak in the engine valley (OM642 3.0 V6 diesel, CLS350 CDI / BlueTEC)
Oil cooler seal leak in the engine valley (OM642 3.0 V6 diesel, CLS350 CDI / BlueTEC) is a common fluid leaks fault on the Mercedes-Benz CLS on the 2011–2017 cars, and we rate it major. Putting it right costs $1,800 to $3,200 at New Zealand independent workshop rates. Below is what goes wrong, the symptoms to notice on a test drive, where to look, and what to ask the seller before you hand over any money. It is one of 5 documented faults on the Mercedes-Benz CLS.
- Vehicle
- Mercedes-Benz CLS
- Engine
- 3.0L
- Years
- 2011–2017
- Fuel
- diesel
- System
- Fluid Leaks
What you are looking at
No photograph available
What goes wrong
The OM642 3.0 V6 diesel has its oil cooler buried in the valley between the cylinder banks, sealed by rubber seals that harden and leak. Oil pools in the valley and runs out the back of the engine, so the first sign is usually wetness at the bottom of the gearbox bellhousing rather than anything visible from the top. Mercedes changed to purple Viton seals around 2010, so leaks are less common on the CLS than on earlier cars but still happen with age. The seals are cheap but the job is 8 to 10 hours because the intake, turbocharger pipework and EGR parts must come off; ignored, it becomes a steady oil loss and a fire and belt-contamination risk.
What you would notice
- Check oil level warning on the dash
- Oil drips or a stain under the join between engine and gearbox
- Burning oil smell after a hard drive
- Oil level dropping between services with no smoke
How to check it
Where to look. Centre of the V between the cylinder banks under the intake; leak shows at the bottom of the gearbox bellhousing
Look under the car with a phone torch at the join between the engine and gearbox (the bellhousing): fresh oil or a black wet film there is the classic sign.
Check the ground where the car is normally parked for drips.
Check the oil level on the dash or dipstick and ask whether it needs topping up between services.
After the test drive, sniff for burning oil.
Ask for invoices showing the oil cooler seals have been replaced and whether the updated seal was used.
If a workshop inspection is possible: on a hoist, remove the undertray and check whether oil is coming from the rear of the engine valley (oil cooler) or from the sump or rear main seal, as the fix and cost differ.
What it costs to put right
$1,800 to $3,200 at New Zealand independent workshop rates. That is for this job on its own, not for the car.
Ask the seller
- “Have the oil cooler seals ever been replaced, and do you have the invoice?”
- “Does the car ever show a check oil level warning?”
- “How much oil does it use between services?”
Now check the Mercedes-Benz CLS you are actually looking at
Everything above is true of the model, and it is free. What it does not tell you is whether THIS car has been looked after: its odometer history and whether the reading has ever gone backwards, whether there is still money owing on it, whether it has been reported stolen, how many owners it has had and what it is really worth today. That comes from the official records for that plate. Enter the plate for the free details, or get the full history and a market valuation from $13.49.
Where this comes from
- ID Parts blog: Mercedes Oil Cooler Seal Leak on 3.0L V6 Engines (OM642)
- MBClub UK forum thread: 642 engine oil coolers leaking
- MBWorld forum thread: OM642 Oil cooler leak - DIY fix?
- Fifth Gear: Mercedes-Benz BlueTEC Leaking Oil Cooler
Mercedes-Benz CLS buyer questions
Is oil cooler seal leak in the engine valley (om642 3.0 v6 diesel, cls350 cdi / bluetec) serious on a Mercedes-Benz CLS?
We rate it major and common on the Mercedes-Benz CLS. The OM642 3.0 V6 diesel has its oil cooler buried in the valley between the cylinder banks, sealed by rubber seals that harden and leak. Oil pools in the valley and runs out the back of the engine, so the first sign is usually wetness at the bottom of the gearbox bellhousing rather than anything visible from the top. Mercedes changed to purple Viton seals around 2010, so leaks are less common on the CLS than on earlier cars but still happen with age. The seals are cheap but the job is 8 to 10 hours because the intake, turbocharger pipework and EGR parts must come off; ignored, it becomes a steady oil loss and a fire and belt-contamination risk.
How much does it cost to fix on a Mercedes-Benz CLS?
$1,800 to $3,200 at New Zealand independent workshop rates. That figure is for this job on its own. A car showing the symptoms on this page is usually worth either walking away from or negotiating down by at least the repair figure.
Which Mercedes-Benz CLS years are affected?
Our research records this fault on the 2011–2017 Mercedes-Benz CLS. Build dates move around, especially on imported cars, so treat the range as the years to look hardest at rather than a guarantee either way. The official records tell you the exact build and first registration date for the car in front of you.
How do I check for it before I buy?
Look under the car with a phone torch at the join between the engine and gearbox (the bellhousing): fresh oil or a black wet film there is the classic sign. Check the ground where the car is normally parked for drips. Check the oil level on the dash or dipstick and ask whether it needs topping up between services. After the test drive, sniff for burning oil. Ask for invoices showing the oil cooler seals have been replaced and whether the updated seal was used. If a workshop inspection is possible: on a hoist, remove the undertray and check whether oil is coming from the rear of the engine valley (oil cooler) or from the sump or rear main seal, as the fix and cost differ. Look here: Centre of the V between the cylinder banks under the intake; leak shows at the bottom of the gearbox bellhousing